Been trying to figure out what is CX policy regarding infants and having them sit in their own seat in Y.

Will be traveling with out 18m old on a few short-haul CX flights. Considering flying in Y and buying an extra seat for the little one. Question though, do I need to bring any kind of safety restraint device/car seat for them to sit in their own seat?

Can the parents hold onto them during take-off/landing/seatbelt sign on times, and all other times have them sit in their own seat? Really want to avoid having to bring a car seat...

The other alternative is get J seats and bring them as a lap infant.